The vestibular system produces a number of compensatory responses to accelerations imposed upon the head. Probably the least understood of these are vestibular influences on thoracic and abdominal "respiratory" muscles which also have postural and cardiovascular functions. The goal of this project is to determine both the functional significance of vestibular-evoked respiratory muscle responses and the neural substrate that produces these responses. Experiments will be conducted using the cat, for which there is considerable relevant background information concerning the vestibular and respiratory systems. Two types of experiments will be performed to gain insight into the functional significance of vestibular-respiratory responses, using the decerebrate cat preparation. First, the extent of the responses will be examined. In particular, we will determine if the activity of the muscles of the upper airway, like that of the respiratory muscles of the thorax and abdomen, is influenced by electrical stimulation of the vestibular nerve. Second, natural vestibular stimulation in multiple vertical planes and in the horizontal plane will be used to determine the directional sensitivity and dynamics of the respiratory muscle response. The neural substrate that produces the vestibulo-respiratory response will be examined using three types of experiments. First, the anterograde neuroanatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L) will be iontophoretically injected into the medial and adjacent inferior vestibular nuclei to determine which areas containing respiratory neurons may receive direct vestibular inputs. The medial and inferior nuclei were chosen for study since they are known to be essential for mediating vestibulo-respiratory responses. Second, the responses of medullary respiratory neurons to electrical and natural vestibular input will be determined. Our preliminary data indicate that at least some of these neurons are responsive to vestibular stimulation. Finally, medullary respiratory groups will be inactivated using chinook acid microinjections to determine their importance in mediating vestibulo-respiratory responses.